This invention relates to a refractive index controlled diffractive optical element formed inside a transparent material and a method for manufacturing that element utilizing a diffraction phenomenon between the regions where refractive indices are changed or between the regions where a refractive index is changed and where a refractive index is not changed.
A refractive index distributed optical elements formed in a transparent material with regions of which the refractive index is different from that of a periphery of substrate is used in a wide range of applications including optical communication and optical devices. Above all, graded index lens (GRIN lens) are used as pickup components etc. for CDs and DVDs because an extremely large number of highly integrated alley bodies, that cannot be manufactured from normal spherical lenses, can easily be obtained.
Among these graded index lenses, binary lenses utilizing the light diffraction phenomenon have an advantage of being capable of reducing the size and weight of an application device because of the small thickness of the lens. In particular, it is preferable from the viewpoint of the robustness of the optical element to form refractive index distributed optical elements by multiphoton absorption of a pulse laser beam because the region having a different refractive index can be created in a transparent material.
A method for forming a binary lens inside a transparent material, such as glass or sapphire, is known as described in Patent Reference 1. This is a binary lens that has a plurality of ring-shaped regions, where a refractive index is concentrically changed with a specified pitch inside glass, and ring-shaped regions formed around the former regions, where a refractive index is invariable, are alternately formed. An example that a pulse laser beam scanned inside of the glass at a constant speed is shown above.
Technology for improving the diffraction efficiency of a binary lens or enabling more complicated optical control, as shown in Non-patent Reference 1 is also known. In this technology, a diffractive lens such as a binary lens is manufactured by creating step-wise regions in a transparent material, such as silica glass, where a refractive index is changed by a specified amount, in the direction of travel of diffracted light, which is limited by simulative blazing a diffractive face.    Patent Reference 1: Patent publication number 2003-35809 (paragraphs 0013 through 0015)    Non-patent Reference 1: Yamada, K. et al. Optics let. Vol. 29 No. 16 (2004)
However, in the binary lens shown in the example of Patent Reference 1, a pulse laser beam was scanned inside the glass having a constant laser power at a constant speed, in order to form a binary lens with a region where a refractive index is changed by a certain amount and a region where a refractive index is invariable. Thus, an error in a refractive index distribution is observed in comparison with an ideal curved surface shape as shown in FIG. 1. Therefore, there was a problem that diffraction efficiency of the binary lens as well as performance of the binary lens is limited.
In the technology relating to Non-patent Reference 1, an increase in the number of steps in the region where the refractive index inside a transparent material is changed results in a significant increase in the number of manufacturing processes. Therefore, there was a problem that the time required for manufacturing increases significantly.
In the technology relating to Non-patent Reference 1, when steps in the region where the refractive index is changed are formed inside the transparent material, it was found that the diffractive optical element have different diffraction efficiency for the incidence from the front and reverse sides of the diffractive optical element. Furthermore, there was a special problem that an increase in the number of steps in the region where the refractive index is changed results in a significant shift in the focal length of a binary lens from a theoretical value.
In the technology relating to Non-patent Reference 1, an increase in the number of steps in the region where the refractive index inside the transparent material is changed results in a proportional increase in the thickness of the region where the refractive index is changed, leading to an increase in the thickness of a binary lens itself. Thus, there was a problem that an effect of reducing the thickness, which was the greatest advantage of a binary lens, is diminished.
This problem is present not only in binary lenses but also in entire diffractive optical elements. An attempt to increase the number of steps in the region where the refractive index inside a transparent material is changed to raise the level of blazing in order to improve the diffraction efficiency to a particular order results in an increase in the number of scans by the pulse laser in proportion to the number of the steps. Thus, there was a problem that complicated manufacturing processes significantly increase manufacturing time, leading to an increase in the thickness of formed diffractive optical elements.